The present invention relates to a current probe for ultra high frequency current measurements.
Presently available current probes measure the current flowing through a signal line by being coupled magnetically to the signal line. Some current probes require that the signal line be broken and be threaded through a magnetic core to provide the magnetic coupling. Other current probes do not require that the signal line be disrupted in this manner.
For example, U.S. Pat. No. 5,044,974, issued Apr. 2, 1991 to Cattaneo et al. shows a current probe which includes an assembly on which is mounted an electronic printed circuit board. The assembly includes a magnetic circuit consisting of a stack of metal sheets. The signal whose current is being monitored is sent through a winding around the magnetic circuit. The winding is formed by U shaped conductor portions surrounding three sides of the magnetic circuit in the assembly, and through corresponding conductive traces on the printed circuit board interconnecting the U shaped conductor portions. These traces are laid out to allow the number of windings around the magnetic circuit to be varied. A second winding around the magnetic circuit is used to provide an input to a measuring device. A further provision of a location for a Hall cell within the magnetic circuit allows for the second winding to zero out the net magnetic flux in the magnetic circuit, allowing the device to be used as a nulling sensor. This current probe can be mounted on a main printed circuit board, and operates at relatively low frequencies.
However, all existing ultra high frequency current probes of relatively high sensitivity require that the signal line whose current is being observed be threaded through, and possibly wound around, a closed ferrite high frequency transformer core. In this way, the signal line forms one winding of a transformer. The other winding of the transformer is connected to the test equipment (i.e. oscillo-scope, network analyzer, etc.). In these current probes, the presence of the ferrite core, and the test equipment attached to its winding, causes some amount of change to the electrical characteristics of the circuit. This is because the transformer and the attached test equipment form an electrical element having a finite resistance, and, most likely, a non-zero reactance. Thus, the results of the test do not accurately reflect the operation of the circuit with the current probe removed.
Mechanically, such an arrangement also produces much wear and tear on the circuit being observed. This is because to make a measurement the signal line must be broken, the ferrite core threaded onto the signal line, the signal line reconnected, the measurement made, the signal line broken again, the ferrite core removed and the signal line reconnected again. There is also wear and tear on the current probe itself from this procedure. For production line testing, high volume measurements are performed, often, with automated handling of the product being tested (i.e. by robots). In such an environment, a current probe as described above must be inserted manually in the circuit. The time required for inserting and removing such a current probe is significant.
A current probe which operates at ultra high frequencies, which makes connections between the signal line being observed and the test equipment fast, easy and able to be performed by automated handling, and which can provide a minimal change to the electrical characteristics of the signal being observed is desirable.
In accordance with principles of the present invention an ultra high frequency current probe is fabricated in the form of a surface mount device, designed to be a part of the circuit providing the current being observed.
Because such a current probe becomes a part of the electrical circuit, it has a minimal effect on the electrical characteristics of the signal being observed. Also there is no manual handling of a current probe during testing.